1. Field of the Invention
The present invention relates to a wire bonding method in which a wire passing through a capillary is connected to a first bonding point and second bonding point by the capillary, and more particularly, to an M-shape wire loop formation method.
2. Prior Art
Japanese Patent Application Laid-Open (Kokai) No. H10-189641 discloses, in its column 4, line 22-column 5, line 24 and in FIGS. 2 and 3, an improvement of a trapezoidal wire loop which is disclosed, for example, in FIG. 2 of Japanese Patent Application Publication (Kokoku) No. H5-60657. In this improved wire loop, a part of the upper linear portion of the trapezoidal wire loop is formed in a downward bow shape, so that the wire loop is in substantially an M-shape. Below, this loop shape will be referred to as an M-shaped loop.
The M-shaped loop will be described in detail below with reference to FIGS. 4 through 6.
As shown in FIG. 4, the M-shaped wire loop is formed when upper surface (first bonding point) of a pad 2a on a semiconductor chip 2 mounted on a lead frame 1 and the upper surface (second bonding point) of a lead 1a on the lead frame 1 are connected by a wire 3, and it consists of a neck height part 31, a trapezoid length part 32 and an inclined part 33. Kinks 3a and 3b are formed at both ends of the trapezoid length part 32, and a kink 3c is formed in the trapezoid length part 32 so that the shape of the trapezoid length part 32 is bowed downward. This M-shaped loop is formed by the capillary that is moved as shown in FIG. 5 by the steps shown in FIG. 6.
As shown in steps (a) of FIG. 6, the capillary is lowered with a clamper (not shown) which holds the wire 3 maintained in a closed state, and a ball formed on the tip end of the wire is bonded to the first bonding point A. Afterward, the capillary 4 is raised to point B while the wire 3 is delivered. Next, as shown in step (b), a reverse operation is performed in which the capillary 4 is moved horizontally to point C in the opposite direction from the second bonding point G. As a result, a first kink 3a is formed in a portion of the wire 3. The wire 3 delivered in the process from point A to point C forms the neck height part 31 shown in FIG. 5.
Next, in step (c), the capillary 4 is raised to point D1, delivering the wire 3. Afterward, in step (d), the capillary 4 is moved to point D2 in the direction of the second bonding point G. Then, in step (e), the capillary 4 is raised to point D while the wire 3 is delivered. As a result of these steps (d) and (e), a second kink 3c is formed in the wire 3. The length of wire delivered in the operation from point D1 to point D2 (i.e., the length from the first kink 3a to the second kink 3c) forms the first horizontal wire part 34 shown in FIG. 5.
Next, in step (f), the capillary 4 is moved in the opposite direction from the second bonding point G; in other words, a second reverse operation is performed so that the capillary 4 is moved horizontally to point E. As a result of this operation from point C to point E, a third kink 3b is formed in the wire 3. The wire 3 delivered at this time forms the second horizontal wire part 35 shown in FIG. 5. Next, in step (g), the capillary 4 is raised to point F1 while an amount of wire 3 that will form the inclined part 33 shown in FIG. 5 is delivered; and then the clamper (not shown) is closed. When the clamper is closed, no wire 3 is delivered even if the capillary 4 is subsequently moved. Next, in step (h), the capillary 4 is moved horizontally to point F in the direction of the second bonding point G. Further, in steps (h) and (i), the capillary 4 is positioned at the second bonding point G by being caused to perform a circular-arc movement or by being caused to perform a circular-arc movement and then lowered; as a result, the wire 3 is bonded to the second bonding point.
In the prior art described above, the track of the capillary 4 is complicated, as shown in FIGS. 5 and 6. Since the amount of movement of the capillary 4 is large, a considerable amount of time is required for bonding. Furthermore, as a result of the reverse operation in step (b), the wire 3 is bent from the root (first bonding point A); and then the wire 3 is pulled in the direction of arrow in FIG. 7 so that the root portion of the wire 3 (at the first bonding point A) is returned to its original position in steps (h) through (i), and damage such as cracking, breaking, etc. may occur in the neck part 30 of the wire 3 as shown in FIG. 7.
Furthermore, in the M-shaped wire loop, since the elasticity of the wire loop shape as a whole is large, there may be cases in which the kinks 3a, 3b and 3c are pulled and extended in steps (g) through (i), resulting in that the neck height part 31 extending from the first bonding point A to the first kink 3a cannot return to the original perpendicular position; and this causes differences in the wire loop shape and in the height Ha of the kink 3a. In some cases, the height Hb of the kink 3b becomes higher than the height Ha as a result of such fluctuations in the height Ha, or as a result of rebound due to plastic deformation of the wire 3 at the time of bonding to the second bonding point G or of the mold flow that occurs in subsequent processes following wire bonding, etc. As a result, the height Hb of the kink 3b is approximately 200 to 400 .mu.m. Thus, it is difficult to form a wire loop with a stable overall height of 200 .mu.m or less.